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SSR H-bridge motor control


New Member
Good afternoon, I have little knowledge in electronics but I'm posting on behalf of my teammate.

We have been given the task to design and build an electric wheelchair to satisfy a brief for a competition we are in. We have to build everything from scratch in a limited time window so everything we use must be available in flat pack form and must be fairly quick to build up.

Initially we were using mechanical relays without the ability to freewheel, but this was unsafe as it would throw you out of the chair - literally.
So we implemented freewheeling but then we were having major issues with the relays fusing from the back emf and large spikes being caused.

We are now using two MY1016 36V 350W DC motors to drive the chair and have a H-bridge setup for each motor, using 40A SSRs. We've got a flyback schottky diode across each relay and a TVS across each motor to help handle the back emf, and a PWM on the switching side of the relay to control it. On paper it all appears okay, but 4 of the relays blew the other day when testing. One theory is that they all shorted through the heatsinks attached to the diodes coming off the relays, as the plates on the back of them isn't isolated from the workings of the diode, so makes all of the heat sink a live part, we taped them up, but some faces were exposed. Sorry if this sounds brief, and I'll get a diagram up ASAP, but does anyone have any experience with any of this that could give some advise?

We are running them with 36V and at times we will be achieving max torque with the motors. They pull 12.5A each when under max load.

Many thanks, Andy.

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